The 127-page draft, obtained by The Associated Press, paints a harsh warning of what's causing global warming and what it will do to humans and the environment. It also describes what can be done about it.
"Continued emission of greenhouse gases will cause further warming and long-lasting changes in all components of the climate system, increasing the likelihood of severe, pervasive and irreversible impacts for people and ecosystems," the report says. The final report will be issued after governments and scientists go over the draft line by line in an October conference in Copenhagen.
Depending on circumstances and values, "currently observed impacts might already be considered dangerous," the report says. It mentions extreme weather and rising sea levels, such as heat waves, flooding and droughts. It even raises, as an earlier report did, the idea that climate change will worsen violent conflicts and refugee problems and could hinder efforts to grow more food. And ocean acidification, which comes from the added carbon absorbed by oceans, will harm marine life, it says.
Without changes in greenhouse gas emissions, "climate change risks are likely to be high or very high by the end of the 21st century," the report says.
In 2009, countries across the globe set a goal of limiting global warming to about another 2 degrees Fahrenheit above current levels. But the report says that it is looking more likely that the world will shoot past that point. Limiting warming to that much is possible but would require dramatic and immediate cuts in carbon dioxide pollution.
The report says if the world continues to spew greenhouse gases at its accelerating rate, it's likely that by mid-century temperatures will increase by about another 3.6 degrees Fahrenheit (2 degrees Celsius) compared to temperatures from 1986 to 2005. And by the end of the century, that scenario will bring temperatures that are about 6.7 degrees warmer (3.7 degrees Celsius).
"The report tells us once again what we know with a greater degree of certainty: that climate change is real, it is caused by us, and it is already causing substantial damage to us and our environment," Pennsylvania State University climate scientist Michael Mann wrote in an email. "If there is one take home point of this report it is this: We have to act now."
John Christy of the University of Alabama, Huntsville, is in the tiny minority of scientists who are skeptical of mainstream science's claim that global warming is a major problem. He says people will do OK: "Humans are clever. We shall adapt to whatever happens."
While projections show that the world will warm and climate will change, there's still a level of uncertainty about how much, and that makes the problem all about how much risk we accept, said MIT climate scientist Kerry Emanuel.
If it's soon and only a little risk, he said, that's not too bad, but when you look at the risk curve the other end of it is "very frightening."
The report used the word risk 351 times in just 127 pages.
Intergovernmental Panel on Climate Change: http://www.ipcc.ch/
Seth Borenstein can be followed at http://twitter.com/borenbears